An exemplary nuclear reactor, such as a boiling water reactor (BWR), includes a reactor core disposed in a reactor pressure vessel, with the pressure vessel being in turn disposed in a containment vessel. The reactor core includes a plurality of nuclear fuel rods configured in bundles to generate heat which is transfered to water recirculating therein for generating steam to power a steam turbine generator, for example.
Reactivity in the core is typically controlled by a plurality of control rods typically extending vertically upwardly therein from the bottom of the pressure vessel. Conventional control rod drives (CRDs) are mounted below the pressure vessel in a lower drywell region of the containment vessel for selectively inserting the control rods into the core and withdrawing the control rods therefrom for controlling reactivity. The lower drywell is defined by an annular pedestal which is used to support the pressure vessel and which also supports a rotatable platform below the CRDs for their servicing during a maintenance outage. The pedestal must, therefore, support the substantial weight of the pressure vessel, as well as accurately support the CRD servicing platform for the proper removal and installation of the CRDs during the maintenance outage.
The containment vessel is typically a concrete structure having an inner steel liner designed for containing gas of the expected elevated pressure therein in the event of a nuclear accident, as well as preventing significant nuclear radiation release therefrom.
In the event of a severe accident in which the reactor core melts to form hot, molten core debris known as corium, the corium must be suitably contained within the containment vessel without appreciably damaging the containment vessel or the reactor pedestal. In a conventional design, the lower drywell includes a thick slab of concrete disposed directly on the containment vessel floor which is sized for preventing the corium from breaching the liner of the containment vessel floor. During such an accident, the corium released from the pressure vessel will erode the slab until its heat is dissipated and it solidifies. Accordingly, the slab must be suitably thick to contain the corium until it cools. However, the protective slab, therefore, increases the size of the containment vessel, which in turn increases plant costs.
Furthermore, since the slab is disposed on the liner of the vessel floor, subsequent inspection of the liner is impractical, if not impossible, since the slab would have to be removed for such inspection. And, a suitable sump must still be provided in which any leaking water may accumulate for subsequent removal.